Soilconsistence
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  • The Field Book for Describing and Sampling Soils describes the specific force that corresponds with each class. A 2.5 to 3.1 cm cube should be used or a plate 1.0 –1.5 cm long by 0.5 cm thick.
  • The capacity of soil to adhere to other objects such as your fingers. Estimated at moisture content that displays maximum adherence between thumb and fore finger.
  • Non-Sticky – little or no soil adheres to fingers after release of pressure. Sands typically fall in this category Slightly Sticky – soil adheres to both fingers after release of pressure with little stretching on separation of fingers.
  • Plasticity classes; Non-Plastic – will not form a 6 mm wire, or if formed , can not support itself if held on end Slightly Plastic – 6 mm wire supports itself, 4 mm wire does not Moderately Plastic – 4 mm wire supports itself, 2 mm wire does not Very Plastic – 2 mm wire supports itself

Soilconsistence Soilconsistence Presentation Transcript

  • Consistence and consistency Sadia Bibi Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan 2010
    • Important from both soil science and engineering point of view
    • Three terms
    • Strength
    • Consistence
    • Consistency
    • Consistence term is used by soil scientists consistency by engineers
  • WHAT IS Consistence
    • Soil consistence is a physical property to describe the resistance of a soil to mechanical stresses or manipulations at various moisture contents
    • OR
    • Soil consistence provides a means of describing the degree and kind of cohesion and adhesion between the soil particles as related to the resistance of the soil to deform or rupture.
    • Since the consistency varies with moisture content and clay minerals, the consistence can be described as dry consistency, moist consistency, and wet consistency.
    • Consistency that is evaluated includes rupture resistance and stickiness.
    • The rupture resistance is a field measure of the ability of the soil to withstand an applied stress or pressure as applied using the thumb and forefinger.
    • Consistence is commonly determined in the field by feeling and manipulating the soil by hand
    • Observations are made on the amount of force needed to crush the clod and on the manner in which the soil responds to force
    • The degree of cementation of the soil by such materials as silica, calcite, or iron is also considered in identifying soil consistence
    • Cohesion is the attraction of one water molecule to another resulting from hydrogen bonding ( water-water bond).
    • Adhesion is similar to cohesion except with adhesion involves the attraction of a water molecule to a non-water molecule (water-solid bond).
  • Consistency
    • Consistency is used to describe the degree to which a soil resists deformation when a force is applied
    • Consistency is determined by the soil’s resistance to penetration by an object while the consistence describes resistance to rupture
    • Instead of crushing a clod of soil, the engineers attempts to penetrate it with either the blunt end of pencil or a thumb nail
  • Soil strength
    • Most imp. Property of a soil for engineering uses
    • This is a measure of the capacity of soil mass to withstand stresses without giving away to those stresses by rupturing or becoming deformed
    • Soil consistence in general terms can be described as:
    • Categories are associated with increasing moisture content
    • Hard or harsh consistency
    • At low moisture content soil is hard or harsh
    • If tilled clods formation
    • Friable consistency
    • As moisture content increases friability (ease of crumbling of moist soils)
    • For tillage these are optimum conditions
    • range from firm to loose
    • As moisture content increases still further stickiness, plasticity, viscous
    • But when plough under this condition puddling
  • Air dry consistency
    • In air dry state consistency is measured by soil’s resistance to rupture or to fragmentation when squeezed. So it is characterized by
    • Rigidity
    • Brittleness
    • Max. resistance
    • Tendency to crush to powder
    • Inability of crushing material to cohere again when pressed together
  • Air dry Consistency 1 Newton (N) = 0.224 lb/ft Class Description Loose (sands) Separate from each other when soil is non coherent Symbol L or LO or lo Soft (<8 N) (loams) Soil crushes under very low pressure/ weakly coherent Slightly Hard (8 to < 20 N) Soil material crushes- low pressure - little resistance Mod Hard (20 to < 40 N) Soil material crushes- moderate pressure -resistance Hard (40 to < 80 N) ( clay loam) Soil material crushes under strong pressure Break b/w hands Very Hard (80 to < 160 N) (clays) Can not be crushed between thumb and forefinger but b/w hands with difficulty Extremely Hard Pressure applied by foot with full body or with the help of hammer so depends upon cementation
  • Wet Consistency
    • Describe Stickiness-
      • The capacity of soil to adhere to other objects
      • Estimated at moisture content that displays maximum adherence between thumb and fore finger
    • Describe Plasticity
      • Degree a soil can be molded or reworked causing deformation without rupturing. We take soil material in wet condition and roll it b/w thumb and fore finger and then observe whether rod or wire is formed
  • Stickiness Classes
    • Non-Sticky – little or no soil adheres to fingers after release of pressure
    • Slightly Sticky – soil adheres to one finger but other remain clean after release of pressure with little stretching on separation of fingers
    • Moderately Sticky – soil adheres to both fingers after release of pressure with some stretching on separation of fingers
    • Very Sticky - soil adheres firmly to both finger and thumb after release of pressure with stretches greatly on separation of fingers
  • Non-Sticky Very Sticky Moderately-Sticky
  • Why Plasticity?
    • Water Content Significantly affects properties of Silty and Clayey soils (unlike sand and gravel).
    • Plasticity property describes the response of a soil to change in moisture content.
    • – Strength decreases as water content increases
    • – Soils swell-up when water content increases
    • – Fine-grained soils at very high water content possess properties similar to liquids
    • – As the water content is reduced, the volume of the soil decreases and the soils become plastic
    • – If the water content is further reduced, the soil becomes semi-solid when the volume does not change
  • Plasticity Classes
    • Non-Plastic – will not form a 6 mm dia, 4 cm long wire, or if formed , can not support itself if held on hand
    • Slightly Plastic – 6 mm dia, 4 cm long wire supports itself, 4 mm dia, 4 cm long wire does not
    • Moderately Plastic – 4 mm dia, 4 cm long wire supports itself, 2 mm dia, 4 cm long wire wire does not
    • Very Plastic – 2 mm dia, 4 cm long wire supports itself
  • Moist consistency
    • It is a midway b/w air dry consistency and field capacity
    • It is resistance to squeezing forces b/w thumb and fore finger
  • Moist Consistency 1 Newton (N) = 0.224 lb/ft Moist comments Stress Loose Non coherent 0 Very Friable Soil material crushes very easily under very gentle pressure < 8 N Friable (best) Soil material crushes under gentle pressure 8 to < 20 N Firm Under moderate pressure b/w thumb and forefinger 20 to < 40 N Very Firm Strong pressure is required 40 to < 80 N Extremely Firm Can’t crush 80 to < 160 N
  • Atterberg limits
    • Behavior of soil at different water contents is called Atterberg limits
    • Before building roads and motorway is considered
    • The state of consistency b/w solid and liquid range has been divided into a number of distinct stages by imposing limits to indicate the soil water content limits for various states of consistency.
    • Depending on the water content of the soil, it may appear in four states:
    • solid
    • semi-solid
    • plastic
    • liquid.
    • In each state the consistency and behavior of a soil is different and thus so are its engineering properties. These limits were created by Albert Atterberg , a Swedish chemist. They were later refined by Arthur Casagrande
  • Atterberg Limits
    • • Liquid Limit (LL) Upper limit of plasticity of soil
    • is defined as the moisture content at which soil begins to behave as a liquid material and begins to flow but posses a small shearing strength.
    • At this point water film becomes so thick that cohesion is decreased and soil mass flows under an applied stress
    • • Plastic Limit (PL ) is the state of minimum water contents & defined as the moisture content at which soil begins to behave as a plastic material. At that state soil has the property of ploughing. Plastic limit is determined by finding the water contents at which the soil rolled into thread 3mm dia. Begins to break.
    • • Shrinkage Limit (SL) is defined as the moisture content at which no further volume change occurs with further reduction in moisture content. (SL represents the amount of water required to fully saturate the soil (100% saturation)
  • Atterberg limits
  • Plastic Index (PI)
    • • Plasticity Index (PI) is the difference between the liquid limit and plastic limit of a soil
    • By increasing the clay content and O.M. we can increase the plastic and liquid limit
    • Bigger surface area plasticity more
  • Uses
    • The knowledge of the soil consistency is important in defining or classifying a soil type
    • For farming operations e.g., tillage
    • The soil consistency is a practical and an inexpensive way to distinguish between silts and clays
    • Construction purposes
    • On organic soils we cant make roads which have high plastic and liquid limits